One's Dark Self
by original-jade
Summary: Rin's always had Kurikara to fully seal his abilties. But when Amaimon snaps the sword, Rin goes full out demon. Can Yukio and the other exorcists get their friend back before it's too late?
1. Prolouge

Rin was thrown back from Amaimon with tremendous force. Yukio struggled against the goblins that pinned him against a tree while he could only watch as his brother is thrown around like a toy.

"What the hell do you want, Amaimon? I've taken care of you before and I'll do it again!" The half-demon spat blood from his mouth as he gripped Kurikara even harder.

"I have a job to do now. Before I was just playing. You haven't seen my strength." A pillar of Earth flew up from underneath Rin, throwing him straight up in the air. The sword was knocked from his hand and Amaimon rushed to get it.

"Rin!" Yukio yelled to his brother in panic. The goblins tightened their hold.

"Don't try to stop me. It's only a matter of time." Amaimon slammed the sword hilt deep into the soil as Rin fell to the ground. The boy struggled to his feet and rushed toward the Earth King.

"Hey! What're you trying to—" His words were cut off as the demon snapped the hilt off the blade. The flames that surrounded the sword flew back to Rin. His body convulsed as the power flew into him and he let out an unearthly scream.

"Well, this is more interesting than Father said it would be. I look forward to see what else is in store." The demon seemed unfazed by the torture that his half-brother was going through. The screams continued as he became more and more demon like. His teeth became more like fangs, his nails became claws, and his eyes became a dull red.

"R-Rin?" The younger twin hesitantly called to the older. The boy turned at his name and walked toward the tree where Yukio was being held. As he went on, it was clear something was wrong. He didn't actually walk—he half-walked, half-stumbled over to his brother. He stopped about a foot away and looked back at Amaimon.

"I see now. You want to leave your mark. Go ahead, just don't kill him." Rin turned back to Yukio after hearing what Amaimon had to say. He swung his arm back and brought it down across his brother's body.


	2. Chapter 1

Yukio looked over his students and noticed Shiemi was missing. It wasn't like her to be late. He walked over to Izumo who was in her normal class.

"Have you seen Shiemi today?" The girl shook her head.

"She wasn't in class today, either. Maybe something happened at the shop." She quickly returns to the book she was reading. Just as Yukio was about to speak again, the door to the class burst open. A very disheveled Shiemi was leaning on the doorway.

"Shiemi! Are you ok?" She nodded and muttered something no one could hear. Yukio walked over to her. "Hey, what's wrong?" She looked at him with a wild expression.

"I…I saw him…Rin…" The room went silent. Rin had been missing for 6 months and only Yukio, Mephisto, and Shura knew why. Everyone else just knew that he had disappeared. The shy girl continued. "I had to run an errand for Mother, so I went into the town. I guess I lost track of time because it was already noon by the time I got what I needed. On the way back, I saw him. He…he wasn't alone though…" Fear had edged its way into her voice.

"Who was he with?" Yukio was growing concerned. Rin shouldn't have been with anyone.

"It…it was…Amaimon…and he didn't try to leave at all! It almost like Rin was with Amaimon willingly…He didn't have his sword either…" Yukio froze. He strode over to his desk and shuffled through a few of his papers. Finding the one he wanted, he looked up.

"You guys can have the day off. I need to find him." No one moved. Instead, they just looked at their teacher. Konekomaru stood up.

"I want to help find him, too. I might not be that strong, but I want to do what I can." He stared at Yukio with determination.

"If you want to, then you can help. Just know that he might not be the same Rin you remember. I'll meet you outside." This time, everyone headed out the door. When the room was empty, Yukio opened one of the desk's drawers to reveal the hilt of Kurikara. He closed the drawer and sat down with a sigh. _Rin, why did you have to fight Amaimon?_ His thoughts drifted back to the last time he saw his brother.

_Rin's claws raked across Yukio's body leaving four bloody gashes. The goblins let go of his arms as he cried out in pain. Rin just watched his brother. He looked at the blood on his hand with an intense curiosity. Letting his curiosity get the better of him, Rin licked some of the blood of his fingers. The boy's eyes instantly went from a dull red to a bright, vibrant red. He tried to slash at Yukio's neck, but Amaimon pulled him away._

"_Oh dear. I told you not to kill him, didn't I? We'll have to work on your self control." Amaimon dragged his half-brother away and into a portal to Gehenna. Right before they disappeared, he turned to Yukio. "I doubt you'll ever see the brother you once knew." The gate closed. Yukio was able to call Shura briefly before passing out from blood loss._

A loud rattling sound broke Yukio away from his memories. The air vent cover had been kicked to floor. Someone had jumped out of the vent. Before he got a clear view of who the intruder was, Yukio had jumped out from behind his desk and aimed his gun right at them. He didn't get a chance until after he was pinned against the wall with the gun knocked to the floor.

"Where is it?" A flat voice broke the silence. Yukio stared in both confusion and horror at his assailant. Rin was attacking him _again_.

"Rin…where have you been? And what are you talking about?" The older brother responded by tightening his hold on Yukio's wrists.

"You know exactly what I'm talking about. Now, where is it?" Rin's dull eyes showed no emotions at all. Yukio shook his head.

"No, I _don't_. What do you want?" The younger twin glared at his brother.

"The hilt. I know you have it." Rin let go of Yukio and walked away from him.

"You won't find it h—" Yukio lowered his arms and cut his hand on an old nail. Rin stopped and looked back, his eyes starting to glow again. It was obvious that he was trying to control his instincts. Losing the internal battle, he launched himself at Yukio. The younger brother didn't have time to duck, but he didn't have to. Amaimon had jumped out of the vent and grabbed Rin's collar.

"Ah, still struggling again I see. Of course, your blood was the first one that he tasted, after all. It's only natural that his control would weaken upon sensing it. Now, I can hold him here all day, but we came for the hilt. Where is it?" The demon king held out his other hand. Yukio glared at him.

"I don't have it! And even if I did why would I give it to you?" Amaimon shrugged and let go of Rin. The crazed boy jumped at Yukio, clawing his arm and leaving a deep gash. Yukio let his injured arm hang as he tried dodging the other attacks. The door rattled, causing the three of them to look in that direction.

"Yuki-kun, are you ok? We heard some noises and—" Shiemi had poked her head in the door but froze upon seeing Rin and Amaimon.

"Shiemi! Get out of here!" Seeing Yukio's reaction, Amaimon signaled to Rin. Rin's eyes stopped glowing and he wordlessly grabbed Shiemi. He pinned her against the wall in a chokehold.

"Ah…Rin…what're…" Her words were cut off as Rin tightened his grip. Amaimon held out his hand again. It was clear he wasn't going to take no for an answer. Instead of giving in, Yukio grabbed Rin's tail and pulled it as hard as he could. Crying in pain, Rin dropped Shiemi and whirled on Yukio. He hadn't noticed how close he had pushed Yukio towards the forgotten gun until a loud bang resonated throughout the huge room. Amaimon rushed forward and caught the boy. There was a lot of blood seeping into the side of Rin's shirt where he'd been shot.

"You're lucky he'll heal. Otherwise, I might've had to take things into my own hands. This isn't over." Amaimon carried Rin through a portal back to Gehenna. Yukio tried to stop them, but the portal closed as quickly as it had opened. Shiemi broke the silence as everyone else came back into the partially destroyed room.

"Yuki-kun! You're hurt! And what was all that?" Yukio turned away from her and opened one of his desk drawers.

"Six months ago, Rin and I had chased some rouge hobgoblins into the forest. The further we chased them, the more we saw. They had grouped together in the middle where Amaimon was waiting. We took out as many goblins as we could, but Rin decided to take on Amaimon himself. There was too much of a difference between their powers—Amaimon easily overpowered him." He paused as he placed the hilt on the top of the desk. "He snapped the sword and took Rin to Gehenna. I don't think he's the same idiot we used to know."

"I know we can get him back! He can't just be gone." Shiemi defiantly stated as she tried to take care of Yukio's arm. "Now, take off your coat so I can deal with this." Yukio hesitated before pulling it off.

"There might be a way, but it'll take a lot of sacrifice." He sighed. Suguro slammed his hand down on one of the tables.

"But that's no reason to give up! Like you said, the guy's an idiot, but that doesn't mean we don't need him back." He stared hard at Yukio.

"You don't get it! He's not…he's not Rin anymore! He's more of a monster…Believe me, I know…" The boy let his words trail off as he remembered the day Rin left. Shima stepped up beside Suguro.

"I kind of agree with Bon on this one. What could make him a monster anyway? I don't think he's capable of hurting us, is he?" The once confident words sounded quieter than Shima intended.

"I know you saw what he did to Shiemi…and that…that didn't even come close to what he did before…" Everyone stared at their young teacher as he showed them the scars Rin had left behind. "He didn't say anything, he just attacked. This is why I didn't want you involved. He's too dangerous now."

"Well, I don't care. He's too destructive to be on the demon's side. We need him back and there's nothing you can do to stop us from helping." Izumo glanced around before leaving the room.

"She's such a hothead…and that's why she's so cute…" Shima muttered as everyone else just stared at him.

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**A/N**

Sooooo many reviews, faves, and follows! Thanks guys! Sorry it took so long. I just got kinda busy (and addicted to Attack on Titan (눈‸눈)). But I'm back now and that's what matters! It might be a few weeks before I get the next chapter up so just keep watching for it!


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